1. Field of the Invention
The present invention relates to a zoom lens system and, in particular, to such zoom lens system which, in the autozooming mode thereof, is capable of automatically changing an angle of view according to camera-to-subject distances to thereby provide a subject of a constantly given size on a photographing screen.
2. Description of the Related Art
In an ordinary zoom lens system used in a still camera or in a video camera, in order to achieve a zooming operation, part of lenses forming such zoom lens system is moved to thereby vary a focal distance and thus vary an angle of view.
Also, in recent years, there has been proposed an automatic zoom lens system which is capable of automatically varying an angle of view according to camera-to-subject distances to thereby photograph a subject in a constantly given size with respect to a photographing screen. Examples of such automatic zoom lens system are disclosed in Japanese Patent Publication No. 60-1602, Japanese Patent Application Laid-open (Tokkai) No. 60-249478, Japanese Patent Application Laid-open (Tokkai) No.61-10372, Japanese Patent Application Laid-open (Tokkai) No.61-288210 and the like.
However, there are still left problems to be solved in the above-mentioned prior art zoom lens systems. Specifically, in these prior art systems, in order to be able to photograph a subject in a given size regardless of camera-to-subject distances, the distance from a camera to the subject must be first measured by means of distance measuring means, an angle of view is found by operating the thus measured distance and a previously set value of a size which the subject occupies in a screen, and the zoom lens in driven by means of a zoom drive system in accordance with the thus obtained angle of view.
Therefore, in the prior art automatic zoom lens system, there are required a memory for a data table which must be set up previously, a complicated control system exclusively used for the automatic zoom lens system, and means for detecting of positions, which makes the structure of the system complicated.